As described in Patent Document 1, methods of machining screws include a method of forming a thread on a work material by cutting and subsequently plastically deforming a surface of the thread by rolling. Specifically, when a screw is formed on an inner circumferential surface of a cylindrical body used as a work material, a cutting tap is used for forming a female screw on the inner circumferential surface of the cylindrical body and a rolling tap is then used for plastically deforming a surface of a thread of the female screw so as to form a female screw with an effective diameter.
A screw formed by using this method of machining a screw has a thread surface work-hardened so that a predetermined strength can be achieved.